This invention relates to color cathode ray tubes and more particularly to an improvement in the process of forming a patterned cathodoluminescent screen on the viewing area of a cathode ray tube face panel.
Cathode ray tubes, of the types utilized to present color display imagery for television and allied applications, usually employ patterned multi-element screen structures comprised of repetitive groupings of related cathodoluminescent color-emitting phosphor materials. As practiced in conventional tube construction, a discretely apertured pattern member is usually positioned in spaced relationship with the patterned screen. In a post deflection type of tube, this apertured member functions as an electrode in the finished tube, and is commonly utilized in the prior deposition of the patterned elements of the cathodoluminescent screen on the interior surface of the glass viewing panel portion of the tube envelope. In the common shadow mask type of tube construction, the multi-element screen pattern is similarly formed by using a spacially positioned apertured pattern member. In both types of tubes, each of the openings or apertures in the pattern member, being of a substantially round, ovate, elongated, or rectangular shaping, is related to a specific grouping of similarly shaped related color-emitting phosphor elements spaced therefrom in a manner to enable selected individual electron beams to traverse the common aperture and impinge the proper pattern elements therebeyond. Normally the individual phosphor elements of the screen pattern are separated from one another by relatively small interstitial spacings which tend to enhance color purity of the imagery by reducing the possibility of adjacent color-emitting elements being energized by a specific electron beam.
It has been found that contrast in color screen imagery can be improved by filling in the interstitial spacings between the respective phosphor elements with an opaque light-absorbing material. Primarily, the inclusion of this fill-in material enhances contrast by preventing ambient light from being reflected by the unexcited areas of the screen and the aluminum backing on the screen in the interstitial areas not covered by phosphor elements. Thus, by incorporating such material into the screen structure, each phosphor element, as seen by the viewer, is defined by a substantially nontranslucent encompassment which collectively comprise a multi-opening pattern in the form of a windowed-webbing having a lace-like array of opaque interconnecting interstices. Such web-like screen structures have been fabricated, either before or after phosphor screening by several known processes wherein photodeposition techniques constitute a fundamental part.
Usually each phosphor area of the color screen pattern and the electron beam impingement thereon are of areas larger than that of the associated window in the opaque webbing. Thus, there is "extra" phosphor material and extra electron excitation energy that is masked from the viewer and/or absorbed by the opaque webbing at each phosphor site. Thus, the definitive windows in the opaque webbing, while beneficially improving contrast and color purity, tend to reduce luminescent brightness by blocking out and absorbing the peripheral luminance of the formed phosphor areas. This is particularly noticeable in those screen pattern elements which are constituted of phosphor materials that are least bright in luminescence and color emission.
A variety of methods have been employed to form the patterned screen structures for color cathode ray tubes having defined color-emitting window areas; for example, one conventional process makes use of a repetitive photoprinting technique. For this procedure, the tube viewing panel, having the opaque windowed-webbing priorly formed thereon, is coated with a thin film of a negative photosensitive binder substance, such as sensitized polyvinyl alcohol, and a specific color-emitting phosphor material. This coating application is achieveable by several techniques, for example, one process involves the application of a coating film of the photosensitive substance upon which phosphor powder is disposed, while by another procedure, the screening material is applied as a suspension of phosphor in a photosensitive substance. Regardless of how the phosphor is applied, the coated panel is then discretely exposed to radiant energy, in substantially the ultraviolet range, to cause the negative photosensitive substance to light polymerize and adhere to the interior surface of the panel thus binding the phosphor particles therewith. Prior to exposure, the apertured mask member is temporarily positioned within the panel in spaced adjacency to the sensitized coating, whereupon the mated mask-panel assembly is suitably oriented on an exposure apparatus. This apparatus includes means for predeterminately positioning an optical system wherefrom exposure light is radiated and directed through the mask apertures. Discrete areas of the photosensitive film, thusly exposed to the radiant energy, become polymerized or hardened thereby adhering to the surface of the glass panel forming an imprint of a first screen pattern of phosphor elements in the appropriate window areas. The exposed screen pattern is then subjected to a developing step which removes the intervening unexposed portions of the photosensitive film that were shadowed by the solid portions of the mask member during exposure. The above described procedure is twice repeated in a related manner to dispose the required associated color-emitting phosphor elements to complete the patterned screen combination. For the separate exposure of each portion of the screen pattern, the light source and associated optical system are properly positioned to effect deposition of the respective color-emitting components in the proper window areas.
In U.S. Pat. No. 3,697,301, issued to R. L. Donofrio et al. and assigned to the assignee of the present invention, there is disclosure that the inherent brightness efficiencies of the phosphor materials comprising the screen of the cathode ray tube can be utilized by optimizing the thickness of the phosphor elements comprising the screen pattern. Because of the differences in the light attenuating characteristics of the various phosphor materials that are utilized in patterned screen construction, it has been found difficult to adequately achieve efficient photodeposition of patterned elements that are optimized both in thickness and areal dimension. For example, the light attenuation characteristics of some phosphor materials are such that relatively lengthy exposure times are required to effect the degree of polymerization necessary to securely adhere the pattern element to the surface of the panel. In some instances, excessive exposure promotes undesirable lateral or areal polymerization beyond the bounds normally desired to insure good color-purity operation. Thus, in achieving the desired adherence, and the controlled areal polymerized "growth" of the pattern element, the optimum screen weight or thickness becomes a factor sometimes relegated to a secondary degree of importance. An attempt to overcome this type of difficulty has been manifest in the utilization of additional exposure radiation emanating from a front oriented light source projected through the panel in conjunction with the normal exposure radiation directed through the aperture mask. Simultaneous usage of two light sources presents problems for adjusting proper exposure from both units and increases inherent maintenance problems.